Effects of temperature, water content and nitrogen fertilisation on emissions of nitrous oxide by soils

Nitrous oxide emissions were measured from several grassland and arable soils in the field, and from two of these soils and a forest soil transferred in large monoliths to a greenhouse. The effects of fertiliser N additions and of soil water content and temperature were investigated. Emissions were in the order grazed grassland>grassland cut for conservation>potatoes>cereal crops, and generally were higher than those from temperate natural ecosystems. Based on these data, agricultural soils constitute the major soil source of N2O in the U.K. The highest emission recorded was 8 kg N2O–N ha-1 over 10 months, from a grazed grassland site. Emissions varied from year to year, depending particularly on rainfall at the time of fertilisation. When soil mineral N was not limiting, exponential relationships between N2O flux and both water-filled pore space (WFPS) and temperature were observed. The Q10 value for a sandy loam was 1.6, but ranged up to 12 for a clay loam soil at high WFPS. The high values were attributed to the increase in anaerobic zones where denitrification could take place, as respiratory demand for O2 increased. A forest soil (peaty gley) showed an optimum water potential for N2O emission. Diurnal fluctuations in emissions were associated with diurnal cycles in soil temperature, but with varying time lags, which could be explained by the N2O being produced at different depths.